Kennedy's future, along with that of Delon Armitage, has been the subject of speculation in recent months, with Toulon reportedly interested in both players.
And 30-year-old Kennedy, who has made more than 200 appearances for Exiles since his debut in 2002, has now confirmed that he has agreed a three-year deal away from Berkshire.
"London Irish has been a fantastic home for me for the last 10 years and it's with some sadness that I'll leave Sunbury at the end of this season," he said.
"I've only ever played for one professional club in England, so when the time came to look at my options to be honest I couldn't see myself playing at another club in the Premiership.
"I'm looking forward to France, but for now my focus is on the rest of this season with London Irish. We've battled hard to be in the top six of the Premiership and our focus now is to push on and make the play-offs and finish the year on a high."
Kennedy, who will be linking up with Delon Armitage's brother Steffon, also tweeted: "I feel blessed to have spent over a third of my life playing for my local team as my 'job'.
"I've loved every single day I've spent at Irish. Not one day has felt like a job.
"London Irish will always have a special place in my heart and my dad will continue to be their No. 1 fan! I'm looking forward to a new challenge with Toulon.
"Until then all my focus is on ending this season on a high with Irish. Ending the season on a high starts on Saturday with a huge game against Quins. COYI (Come on you Irish)."
Irish head coach Toby Booth added: "We would like to sincerely thank Nick for his contribution to London Irish over the last ten years. He has been a great servant of the club and an important part of its success. We are sad to see him leave but can see that he has an opportunity to progress his career with Toulon. We wish him the very best of luck."